However, mobile-phone messaging — known as short-message service, or SMS — is a newer technology and the government is still developing ways to control it. The biggest service provider, China Mobile, says customers sent 40 billion messages last year.

Despite promises of official candor about SARS, many Chinese are still ready to believe rumors, owing to the government’s long history of lying about or covering up disasters.

Last month, thousands of people fled Beijing and others stripped grocery stores of food after stories spread that martial law might be imposed or the capital sealed off. The uproar prompted city leaders to take the rare step of publicly denying the rumors.

Nationwide, police have detained 105 persons in 17 provinces and major cities on charges of spreading rumors, according to the official Xinhua news agency.

In Beijing, Xinhua said, a man was detained May 3 after posting a comment online that 400 people had died of SARS in a city the agency didn’t identify.

“The rumor caused a very bad social influence,” Xinhua said.

In the eastern city of Nanjing, a man was detained for sending mobile-phone messages that setting off fireworks — a traditional tactic to drive away evil spirits — would stop the virus, said a police spokesman.

“He meant to promote superstitions and disturb public order,” said the spokesman, who refused to give his name.

Other detainees reportedly include teenage pranksters and business people trying to hurt rivals or promote phony cures. Police say most were either fined or detained for up to 15 days.

Police wouldn’t say how people accused of spreading rumors via SMS were caught, though government newspapers hint at high-tech detective work by special police units.

All messages sent from cell phones carry that subscriber’s phone number, and those who send messages through a Web site must log in, said X.J. Wang, a senior analyst with the Yankee Group in Boston.

The sender’s information is much harder to falsify here than with regular e-mail, he said.

“To trace is very easy,” Mr. Wang said, “but to stop it is not so easy.”